1. Field of the Invention
The present invention relates to a waste gate structure of a turbocharger, and more particularly to a waste gate structure for controlling a boost pressure of a turbocharger for a vehicle's engine.
2. Description of the Prior Art
One example for a conventional waste gate structure of a turbocharger is disclosed in Japanese Patent Application Laid-Open Publication No. 54(1979)-19007. As shown in FIG. 4, this conventional waste gate structure includes a housing 102 in which a turbine rotor 101 is disposed and to which an exhaust manifold 103 and an exhaust gas outlet pipe 104 are connected, respectively. At the inlet side of the housing 102, namely, at the side of the housing 102 to which the exhaust manifold 103 is connected, a bypass hole 105 is formed thereon and is opened or closed by a bypass valve 107. The bypass valve 107 is driven by a control device 106 so that the bypass flow of the exhaust gas flowing into a bypass passage 107 is controlled in order to control the boost pressure. The bypass passage 107 is formed in the housing 102 so as to bypass the turbine rotor 101. One end of the bypass passage 108 is opened via the bypass hole 105 into the inlet side of the housing 102 in which the inlet side of the turbine rotor 101 is located. The other end of the bypass passage 108 is opened as an ejecting nozzle 109 into the outlet side of the housing 102 in which the outlet side of the turbine rotor 101 is located. The ejecting nozzle 109 has a circular shape and its opening is oriented along the direction of the main flow of the exhaust gas discharged from the turbine rotor 101. A bypass volute chamber 110 which constitutes a part of the bypass passage 108 is formed in the housing 102 so as to surround the ejecting nozzle 109. The sectional area or the volume of the bypass volute chamber 110 decreases along the rotational direction of the turbine rotor 101 successively, and the bypass involute chamber 110 communicates with the ejecting nozzle 109 running its whole circumference.
When the bypass valve 107 is opened in order to control the boost pressure and part of the exhaust gas discharged from an engine flows into the bypass volute chamber 110 through the bypass passage 108, the exhaust gas in the bypass volute chamber 110 is discharged from the ejecting nozzle 109 along the direction of the main flow of the exhaust gas discharged from the turbine rotor 101.
In the above conventional waste gate structure, there is a following drawback. As mentioned above, the part of the exhaust gas bypassing the turbine rotor 101 through the bypass passage 108 is discharged from the ejecting nozzle 109 in the axial direction so as to be the same as the direction of the main flow of the exhaust gas discharged from the turbine rotor 101. On the other hand, the exhaust gas discharged from the turbine rotor 101 flows spirally in the output side of the housing 102 and the exhaust output pipe 104. Consequently this spiral main flow of the exhaust gas from the turbine rotor 101 interferes with the bypass flow of the exhaust gas from the ejecting nozzle 109 and the exhaust pressure of the turbocharger. Specifically, the exhaust gas pressure of the outlet side of the turbine rotor 101 increases. As a result, the exhaust gas pressure of the inlet side of the turbine rotor 101 increases, and thereby the efficiency of the turbocharger decreases.